240 Volt Circuits and Electric Boxes

[Bob Deroeck]

Hi,

I'm currently wiring my new workshop. While I have some experience wiring 120 volt circuits, my experience with 240 volt circuits is limited to clothes dryers and electric stoves which use 50 amp surface-mounted fittings which are a combined box and outlet. So, I have a few questions on what to use for boxes and outlets (receptacles) for 20 amp and 30 amp 240 volt circuits.

I'm installing dedicated 240 volt circuits for the cyclone and for the table saw. Plus, I'm installing a 240 volt "ring" circuit which will power the jointer, bandsaw and any other future 240 volt machines. The electrical inspector in town recommended using 10-3 with ground wire for the "ring" circuit so that I would have the option of having 120 volt at the 240 volt machines for auxiliary 120 volt services such as a spot light, etc. Does this design makes any sense? How often have you guys seen machines that require both 240 and 120 volt power, coming in on a single cable? In other words, the 240 volt and 120 volt services are integrated, as opposed to the situation where there is a 240 volt cord and plug and a separate 120 volt cord and plug for auxiliary service on a single machine. It seems to me that if there are machines that require an integrated 240/120 volt service then this design makes sense. But, if there are separate 240 volt cord and plug and 120 volt cord and plug on the machine then there is no need for using 10-3 with ground wire on this circuit since I will have plenty of dedicated 120 volt outlets around the shop that can be used for the separate 120 volt plug of the machine. Also, what types of 120 volt auxiliary services have you guys seen on 240 volt machines, besides task lighting?

With the possible exception of the cyclone all of the 240 volt machines will have plugs going into 240 volt outlets. What shape and size electrical boxes do you guys recommend for 240 volt outlets? The boxes will be located within the walls. I'm familiar with the box "fill" requirements of the NEC. I'm just wondering if you recommend larger boxes for 240 volt outlets than those based on NEC minimum "fill" requirements? Note, there will be only a single 240 volt outlet in any box on the 240 volt "ring" circuit.

For example, say I decide to use 10-3 with ground wire to an 240 volt outlet box which will have an outlet but also pass through to other outlet boxes on the "ring" circuit. For "fill" calculations there are 3 wires entering, 3 wires leaving, a ground wire, and the receptacle. I'm not sure if the single 240 volt outlet should count as one or two volumes, but let's assume it counts as 2 volumes. So the total volume is 3+3+1+2 or 9 volumes x 2.5 (for 10 ga) for a total volume of 22.5 in3. So, a deep rectangular plastic box of 22.5 in3 would be adequate. Is this what you guys would recommend using? Or, would you recommend using a larger round or square plastic box? If you recommend using the larger round or square plastic box, will I have trouble finding a cover plate that will fit the round or square plastic box with an opening for a single 240 volt outlet? And, there is also the question of how would I center the single 240 volt outlet in the round or square plastic boxes. I could see this being a problem with the square boxes. Or, is this a situation where I should just go to an electrical supply house instead of Home Depot or Lowes, because the supply house will have the specialty boxes required for 240 volt outlet? Or, will the specialty boxes required for 240 outlets not be available in plastic, but only in steel?

Thanks for any help you can provide.

Bob DeRoeck

[Rod Sheridan]

Hi, my entire shop is wired with a single multi-wire branch circuit, except for a circuit for the cyclone and the lights.

The issue is one of circuit ampacity.

A 20 ampere multi-wire branch circuit is ideal because the 5-20R receptacles fit 5-15 and 5-20 devices (15 and 20 ampere devices).

I ran the circuits to 4" square boxes containing a 5-20R and 6-20R receptacle, which gives me two 240V 20A outlets and 2 X 120V outlets.

The largest machine I have is a 4HP saw/shaper and a 4HP jointer/planer that require the 20A circuit at 240V, the bandsaw is 240V so it's also using the MWB circuit.

The issue is that if you go to 30 Amperes, you cannot use a 5-20R receptacle on the circuit, so you won't be able to plug in "normal" sized 120 volt loads.

Regards, Rod.

[Rollie Meyers]

What are you calling a "ring circuit"? Ring circuits are used in the UK , (It's why they use fused attachment plugs) ,but are illegal here but if you are just running a number of 240V receptacles on one circuit be sure to size you boxes very generous, trying to stuff 10 pounds of manure in a 5 pound bag is not fun & 10 AWG conductors will not be EZ to deal w/ if one gets stingy on box sizing...

For boxes use 4 or 4 11/16 square metal boxes w/ the approp. plaster ring, & to make a clean install add 1/8" to what ever thickness of the goods covering the wall, 5/8" for 1/2", 3/4" for 5/8", this will have the front of the ring flush w/ the surface providing good solid surface to mount receptacles, nothing gets me frothing at the mouth more then "floating receptacles" that cause plastic wall plates to break when inserting a plug.

[Thomas Bank]

On box size, I typically oversize all my boxes. Maybe if you're a contractor buying thousands of boxes it will add up to some substantial savings on your profit sheet, but for a few here and there you're likely to save more in the time spent not messing with things from a "just legal" box.

I'm wondering what equipment the inspector is considering that will need the dual voltage circuit. Although household equipment such as dryers and ranges is very common in dual voltage - hence the need for the four-prong plug to serve two hots and a neutral plus ground - I'm not coming up with anything I've ever seen in a workshop setting that required it. Of all my own 240V equipment - both wood and metalworking equipment: welders, plasma cutter, compressor, tablesaw, planer, drum sander - not a single one is dual voltage. I'm not saying that it doesn't exist. Just that I don't think that it is common enough to be concerned about.

[Note: When I say dual voltage, I mean that a piece of equipment requires both voltages and requires the four-prong plug. Several of the pieces I mentioned can be wired for EITHER 120V or 240V.]

[David Kumm]

If you are running a thirty amp circuit and plan to branch off it for each machine you want to use some fused disconnects to give you some additional overload protection when using smaller motors. That also allows you to run 110 smaller circuits and use 15 and 20 amp recepticles. I have several machines with dedicated lights - more each year as I age_ so running the extra neutral wire to tap into gives you alot of flexibility to reinvent the wheel when you change your mind about what goes where. I run 3 wire + ground for my 3 phase stuff and am always wishing for a neutral. Dave

[Bob Deroeck]

Rollie,

By "ring" circuit I meant it goes along 4 sides of the workshop. But it is only a single circuit with a number of boxes with a 240 volt receptacle in each of them. The circuit ends in a dead end. It does not tie into the beginning leg of the circuit.

The clarify, my 120 volt and 240 volt circuits will use separate wires and separate boxes on separate circuits. No 120 volt circuits will pass through boxes containing 240 volt circuits and vice versa.

You are recommending square metal boxes for the 240 volt boxes. I don't want to have boxes that are "tight". What "design factor" would you recommend applying to the minimum fill requirements of the NEC? For example, if the minimum fill requirements called for 20 in3, would you recommend a "design factor" of, say, 1.5, giving a box that is a minimum of 30 in3? Another way of looking at it, in the example I gave in my original post with a single 240 volt outlet with 10-3 with ground into the box plus out of the box, what size square metal box would you recommend, including depth (if I'm reading NEC correctly the minimum volume box would be 22.5 in3)?

Thanks.


Bob

[Kent A Bathurst]

I ran 10/3 w/g for my 240 circuits, because that way, I could use 20a dual-voltage duplex receptacles [almost everywhere]. Very convenient - 120 scattered all along the runs. I have 5 240 machines on a total of 2 circuits [both 20 amp], and I don't have more than 2 of them on the same circuit running at the same time - and that only occurs with planer + DC - and there is no 120 at that box. I maybe could have used 12/3 - don't know - didn't check - it's easy to change out a breaker + recept if I ever needed 30a, so I just did it that way. I dont' mind running the circuits, but I don't like re-running them because something comes up that I hadn't planned on originally.

On boxes - I used metal - 4" x 4" x 2.5" for all of them. I've got nothing to prove re: my ability to out-muscle 10 ga wire. I'm a weenie. I love the extra room - much easier to get things buttoned up.

[Doug Palmer]

Bob,

The recommendation to pull a 10/3 w/gnd is one I would agree with. By pulling a 10/3 you are pulling your two hots plus a neutral. Some equipment requires a 4 wire source (2 hots, 1 neutral and 1 ground). If your equipment is pre-wired with any 120v features it will require the neutral to function. You can run 3 wire equipment on a 4 wire receptacle by installing the right plug, but you cannot run a 4 wire on a 3 wire receptacle. Better to plan for the worse case. You never know what you might want to plug in a few years from now. At least make your first receptacle a 4 wire.

As for the box size, you can terminate a single 240v with 10/3 w/gnd in a standard handy box without a lot of difficulty (unless you leave your wires excessively long). When you go to daisy chaining them as you are suggesting and you double the number of conductors in the box - it can still be done, but be prepared to get very frustrated and watch that tongue. One choice is to install a 4" square or even the 4 11/16" square box. If it were me, I would choose a a 2" x 4" deep box as you hinted at. There should be enough room to make your terminations and still push it all in the box and put the cover on with out the need to swear. There is nothing wrong with a larger box and you should have no trouble finding the covers. Metal vs plastic is your choice.

[Roy Turbett]

My detached shop has a 100 AMP subpanel off of the house's 150 AMP main. I ran 10/3 wire to four 30 AMP 240v circuits and am glad I did in case I ever want to put in a 3 phase converter. I only wish I had used bigger boxes. I found out years later that the inspector shouldn't have passed me because some of the boxes are overfilled. I run two PM 90 lathes on one circuit, each with its own overload protection. I run a 3 HP dust collector and a 15" planer on another circuit. My unisaw and 5 HP air compressor have their own circuit. I have three 20 amp light circuits. One of them is also connected to a 5 gal water heater that turns on and off with the lights. I also have four 20 AMP 120v circuits for my band saw, jointer, RAS, mitersaw, router table, mini lathe and two coffee pots. I have as many as ten people at a time in my shop and the only time I have popped a breaker is when I used the miter saw while two pots of coffee were brewing. When I first built my shop I thought I would be the only one in it and never thought it would get the use that it does now. I'm glad now that I "over built".

[Thomas Bank]

By pulling a 10/3 you are pulling your two hots plus a neutral. Some equipment requires a 4 wire source (2 hots, 1 neutral and 1 ground).

Not disagreeing with you, but as I said in my post, I have not found any such equipment in my experience. I'm just curious what is out there that is wired like this? I even did some searching and was unable to turn anything up.

[Doug Palmer]

Not disagreeing with you, but as I said in my post, I have not found any such equipment in my experience. I'm just curious what is out there that is wired like this? I even did some searching and was unable to turn anything up.

I do not entirely disagree that there are few acceptions in the typical work shop, but I will give you one example which probably does not apply to most situations. I recently added 5 hp home made cyclone dust collector to my shop. The motor runs on 230 volts and does not require the neutral leg. The motor starter (contactor and overload block) however has a 120 volt coil on it to pick up the contactor to start the motor. Since I have the neutral already ran, I was able to use my existing 230 v plug to run my DC fan without needing a separate circuit for control.

The 3 phase converter would not necessarily require 4 wire, depending on where you installed the converter, but it could if installed at the source end.

Another example is a hot tub. Again not something you need in the shop, but maybe you want to extend the circuit outside at a later date and this receptacle is your closest source.

Most new appliances have some 120v controls or features which require a neutral. Just a few years ago this was not the case. This is a recent trend, I have no reason to think the power tool industry would not follow this trend.

I realize that these are not the norm, but the difference in cost is worth the potential saving later when your plans change. Who knows, you may later give up woodworking (I hope not) and need to turn the shop into an apartment for extra income. From a power tool stand point, I agree that the need would be unlikely today, but am unsure about future trends. That is why I recommend the home-run be 3 wire plus ground. If you would want to use 3 prong plugs and just cap off the neutral until and unless needed, that would make sense as well. But I would definitely pull the wire.

[Thomas Bank]

The motor runs on 230 volts and does not require the neutral leg. The motor starter (contactor and overload block) however has a 120 volt coil on it to pick up the contactor to start the motor.

Interesting. As I said, I was hard pressed to find anything in a shop setting that combined the two voltages.

A hot tub requires a 4-wire GFCI circuit, so I wouldn't go just tapping into your shop's 240V circuit for that!

I'm still not convinced that the 240V/120V equipment is the "new" trend. It basically comes down to a 120V item that needs 240V for a heating element - ranges, clothes dryers... or hot tubs...

And in my case, my property is only zoned for a single family residence. So the workshop will only ever be a workshop.

[Alan Schaffter]

Here is one reason to run a 3 + gnd 220V circuit: Say you want to mount a 120V worklight on a machine wired for 220V, you either need to buy a very rare 220V bulb, run a separate 120V power cord for the light, engage in a dangerous practice and violate code by wiring the 120V light between one hot and gnd, or . . . . . wire the machine with a 4-wire cord and plug so you have a neutral for the 120V feature.

[Ed Labadie]

Not disagreeing with you, but as I said in my post, I have not found any such equipment in my experience. I'm just curious what is out there that is wired like this? I even did some searching and was unable to turn anything up.

Woodmaster machines have two cords, 220v for the motor & 110v for the variable speed drive. Easier for everyone that its done this way, why make the only machine on the market that requires a 4 prong plug.

Ed

[Alan Schaffter]

There is one other option if you use an LED light - many of the little wall wort power adapters work on 120-220V - connect one of those to the 220V wiring in your machine. That is what I did to power the tach unit on my DP.

p.s. I also tried using a diode (half wave recitifier) from Radio Shack to power the 110V light with 220V. The diode wasn't heavy duty enough so when it quit the bulb burned out.